Session L23: Focus Session: Probing and Modifying Materials with Lasers II

Femtosecond laser structuring in dielectrics

Three-dimensional (3D) structuring of glasses, crystals, and polymers by tightly focused femtosecond laser pulses is a promising technique for microfluidic, micro-optical, photonic crystal and micro-mechanical applications [1-4]. The 3D laser micro-structuring of resists is demonstrated by direct laser writing [1] and holographic recording using phase control of interfering pulses [2]. Tightly focused laser pulses can reach dielectric breakdown irradiance without self-focusing when sub-1 ps pulses are used for laser-structuring inside dielectrics. The limiting case of microstructuring, a void recording, can be achieved [3]. The mechanism of void formation has been explained as a result of dielectric breakdown and micro-explosion. The absorption is localized within a skin depth of tens-of-nanometers in the plasma at the focus. This defines an ultimate localization of the energy delivery by a laser pulse. The absorbance reaches 0.6 in a fully ionized solid state density breakdown plasma. The high temperature and pressure buildup is large enough to generate a shock wave (strong micro-explosion). For example, a single 100 nJ laser pulse forms a void under tight focusing conditions even in the high strength sapphire (Young modulus of 400 GPa). It is considered that material fails upon compression rather than tension for which the mechanical failure threshold is by an order of magnitude smaller. This scenario of breakdown by compression is corroborated by numerical modeling of the strong explosion at our experimental conditions. Modification of materials by tightly focused femtosecond pulses opens new material processing routes for inert dielectrics [4] and can possibly be used for creation of new high-temperature and pressure phases inside the volume of irradiated samples. These regions with altered nano-structure have different chemical properties as was found in silica glass, quartz, and sapphire by wet etching of the ``shocked'' regions in aqueous solution of hydrofluoric acid. Current challenges of structural characterization of micrometer-sized volumes of nano-structures materials are discussed. The achievable resolution limits and potential of the fabricated 3D patterns in photonics, micro-fluidics, and sensor applications will be presented. [1] K. K. Seet et al., Adv. Mat. 17, 541, 2005. [2] T. Kondo, et al., New J. Phys. 8, 250, 2006. [3] S. Juodkazis, et al., Phys. Rev. Lett. 96 166101 2006. [4] S. Juodkazis, et al., Adv. Mater. 18 1361 2006.   

Mechanisms of nanoparticles size reduction by laser irradiation

Size reduction of nanoparticles after laser irradiation is well known phenomenon. Two different mechanisms of size reduction have been proposed: 1) the photoejection of electrons from a particle into a solution which caused ionization and Coulomb explosion of the ionized particle. 2) a simple heating-melting-evaporation mechanism. In this report we show that the different mechanisms are working under different experimental conditions, and give the criterion for their applicability. The main experimental parameter responsible for such criterion is the laser energy flow density, I$_{0}$ = E/$\tau $s, where E and $\tau $ are the laser pulse energy and duration, and S is the laser beam cross section. We calculated the critical value for this parameter in case of spherical silver and gold particles. When this parameter exceeds the value of about 10$^{10}$ W/cm$^{2}$, the electron ejection can be started. For nanosecond lasers such energy flow density values can be achieved only with beam focusing, but for pico and especially femto lasers this condition can be realized in different experimental arrangements. When I$_{0}$ is smaller than the critical value, the particle heating-melting-evaporation mechanism is responsible for particle size reduction.   

Novel Nanostructured Materials and Properties by Pulsed Laser Deposition

Pulsed laser deposition has been used to create novel nanostructured materials either as layered or nanodot structure. By controlling thin-film growth kinetics during island growth, we are able to create three-dimensional self-assembled nanodot structures of Ni and ordered L10 FePt in a given matrix. Epitaxial growth and Integration of Ni and FePt on Si(100) substrate was achieved via domain matching epitaxy which facilitated epitaxial growth across the misfit scale. Magnetic properties can be varied by controlling the orientation and coercivity higher than 1.2 Tesla achieved. These results on ordered L10 FePt will be compared with those Ni with practical implications of information storage (1,2). (1) H. Zhou, D. Kumar, A. Kvit, A. Tiwari, J. Narayan, J. Appl. Phys. 94, 4841 (2003). (2) G.R. Trichy, D. Chakraborti, J. Narayan, J. T. Prater, J. Phys. D: Appl. Phys 40, 7273 (2007).   

A study of photoemission using CW and pulsed UV light sources to probe surface slip band structure evolution of single crystal aluminium

We report measurements of photoelectron emission from high-purity single crystal aluminum during uniaxial tensile deformation. A 248 nm pulsed excimer laser was used as a light source and the generated photoemission data was compared with that using a filtered mercury lamp. Time-of-flight curves of photoelectrons generated by pulsed excimer laser irradiation were observed showing a two peaked structure. These two peaks correspond to photoelectrons of two energy levels. It was also found that real time total photoelectron charge increases linearly with strain; and the increment is heterogeneous. Photoemission using low-energy photons is sensitive to changes in surface morphology accompanying deformation, including slip line and band formation. The discontinuity in photoelectron intensity and the heterogeneous surface slip band structure prove the production of fresh surface area is not continuous, which is predicted by a recent dislocation dynamics theory based on percolation process. Except for differences in instrumentation and data analysis, the photoemission data from a filtered mercury lamp and from the excimer laser are comparable. Current studies extend the application of the excimer laser into surface dynamics analysis.   

Self-Positioning Optically Trapped Microspheres For Nanoscale Laser Direct Write

We present a novel method of near-field laser direct-write patterning by incorporating self-positioning optical manipulation of polystyrene microsphere combined with pulsed laser processing. A 532 or 1064 nm CW laser optically traps a water-dispersed microsphere against a polymer substrate using a 2-dimensional Bessel beam trap. The optical scattering force due to the Bessel beam in the propagation direction is balanced by the repulsive interaction near the surface thereby creating an equilibrium spacing between the two, regardless of large scale surface features. A pulsed nanosecond 355 nm laser directed down the same beam path, is then used to ablate or modify the surface below the microsphere. While the pulsed laser has a large spot diameter, the intensity required for material modification is only achieved directly below the sphere due to focusing and near-field enhancement. Using an x-y translation stage, we demonstrate the ability to move the substrate while keeping the bead fixed in the optical trap, but allowing it to maintain its position above the surface. Direct-write nanoscale features are thereby enabled through this process. Characterization of the resulting structures along with advantages and limitations of this technique will be discussed.   

2D patterned GaN$_{x}$As$_{1-x}$ Quantum structures using Ion Implantation and Pulsed Laser Melting

We will present two dimensionally patterned GaN$_{x}$As$_{1-x}$ nanostructures fabricated in a GaAs matrix using nitrogen ion implantation followed by pulsed laser melting and rapid thermal annealing (RTA). The arbitrarily patterned GaN$_{x}$As$_{1-x}$ regions are investigated by ballistic electron emission microscopy (BEEM), a three terminal scanning tunneling microscopy technique. BEEM can image both the surface topography and the local hot electron transport. Using ion implantation through a lithographically patterned mask and varying subsequent processing conditions such as nitrogen concentrations and laser fluences, we have made locally confined GaN$_{x}$As$_{1-x}$ dots. By analyzing BEEM images of the quantum dots, we study giant bandgap bowing effects on the Schottky barrier height. We will also discuss the effects of different implanted nitrogen concentrations, laser fluences and RTA conditions on the conduction band structures of GaN$_{x}$As$_{1-x}$.   

Femtosecond laser-induced black metals

Metals are one of the most commonly used materials in everyday life. One of the intrinsic properties of nearly all metals is that they are highly reflective for electromagnetic waves. Recently, by treating metal surfaces with high-intensity femtosecond laser pulses, we turned highly reflective metals highly absorptive and created, for the first time, ``black metals''. We also investigated the surface features for metal blackening and characterized the spectral responses of the black metals from UV to IR. The black metals promise potential for a variety of technologically important applications.   

Cooling rates and mechanisms of resolidification in short pulse laser processing of metal targets

Short-pulse laser irradiation of a metal target can create conditions for generation of non-equilibrium phases and unusual microstructure in the surface region of the irradiated target. The shallow melt depths produced by the short pulse laser irradiation and the high thermal conductivity of metals can result in very high cooling rates, strong undercooling and rapid resolidification. In this work, the melting and resolidification processes occurring under conditions of extreme heating and cooling rates are investigated in large-scale molecular dynamics simulations. The kinetics of the resolidification process and the microstructure of the surface region are found to be defined by a competition between the epitaxial regrowth of the substrate and nucleation of crystallites within the undercooled melted region. The dependence of the final microstructure of the surface region on the irradiation conditions is discussed.   

Modeling of early-stage plasma during femtosecond laser ablation of metals

We developed a model of early-stage plasma induced by intense femtosecond laser ablation of metals in an ambient gas. We consider a 100 fs FWHM, 800 nm wavelength laser pulse irradiating a copper target in 1 atm nitrogen environment. Electron and lattice temperature of laser-irradiated target were calculated based on a two-temperature model, with surface electron emission due to thermionic and photoelectric effects utilized as the boundary condition for plasma initiation. Plasma development was calculated based on conservation laws for electrons, ions, as well as atoms from ambient gas. Inverse Bremsstrahlung laser absorption by electrons and electron impact ionization were found to be responsible for plasma development, and the simulation results yielded the laser intensity threshold for femtosecond laser-induced plasma formation.   

Laser-Ablation Deposited Hafnium-Oxide Films for Triple Point Cathodes

The triple-point is defined as the interface between metal, dielectric and vacuum; it provides a copious source of electrons for cold-cathodes. Pulsed-laser-deposition has been utilized to fabricate triple-point cathodes consisting of hafnium-oxide film-islands deposited over metal substrates. A 600 mJ, 20 ns KrF laser ablates a solid target of hafnium metal in a background gas of 20 percent O2 and 80 percent Ar at 100 mTorr at 10-15 pps. Contact lithography is employed to fabricate arrays of Hf-oxide islands on substrates to maximize the area of triple points for electron emission. For materials analysis, the films are deposited on a Si substrate. Plasma plume diagnostics include gated optical emission spectroscopy; neutral and singly-ionized hafnium have been measured. Hafnium-oxide film diagnostics include XEDS, SEM, TEM, profilometry, ellipsometry and x-ray diffraction (XRD). Hafnium-oxide deposition rates are about 0.06 nm/pulse. Cathode experimental current results will be presented at -300 kV.   

Growth control of GaAs nanowires using pulsed laser deposition with arsenic over-pressure

Using pulsed laser ablation with arsenic over-pressure, the growth conditions for GaAs nanowires (NWs) catalyzed by gold nanoparticles have been systematically investigated. The single-crystal structure and geometry of the NWs have been characterized for various growth conditions. Arsenic over-pressure with As2 molecules was introduced into the system by thermal decomposition of polycrystalline GaAs to control the stoichiometry and shape of the NWs during growth. GaAs NWs exhibit a variety of geometries under varying arsenic over-pressures. Without As2 over-pressure, branched growth of GaAs with uncontrollable size and geometry was observed due to the decomposition of GaAs NWs, producing metallic Ga which serves as catalysts for the branched growth of GaAs on the nanowire surfaces. Under optimal As2 over-pressure, at substrate temperature of 570 ?C, single-crystal GaAs NWs with uniform diameter of $\sim $50 nm, small diameter distribution, length over 20 micrometers, and thin surface oxide layer of $\sim $0.5 nm were obtained. X-ray diffraction results confirm the zinc-blende crystal structure of the GaAs NWs. A preliminary electrical characterization gives a linear $I$--$V $curve with a reasonable resistance, which leads to more thorough electrical characterization on GaAs NWs and device fabrication.   

Phase-locking in Y-coupled Quantum Cascade Lasers

A variety of spectroscopic applications call for powerful coherent light sources in the mid and far infrared spectrum [1]. In the past decade this demand has promoted quantum cascade lasers (QCLs) to become crucial light sources for sensing chemical components in the gaseous and liquid phase [2]. Waveguide coupling has pushed forward major developments to fulfill the demands of today's spectroscopers, such as high power output, stable single longitudinal mode operation, narrow spectral linewidth, and frequency tunability. In this work, a monolithic coupling scheme in which two active waveguides merge into a single waveguide is presented for GaAs/AlGaAs quantum cascade lasers [3]. The evolving fields interfere and a constant phase is observed in the Y-shaped laser cavity, resulting in a far field profile of a double slit. The mode distribution is comprehensively derived by matching the farfield profiles to simulated values and shows a weak temperature and current dependence. The concept enhances the output power of a single facet coherent mid-infrared emitter and opens possibilities for monolithic interferometric sensing devices. [1] F. K. Tittel et al., Top. Appl. Phys. 89, 445(2003). [2] G. Wysocki et al., Appl. Phys. B 81, 769 (2005). [3] L. K. Hoffmann et al., Appl. Phys. Lett. 91, 17 (2007).